Experimental investigation of the radial structure of energetic particle driven modes

TL;DR

This study investigates how the radial structure of Alfvén eigenmodes and energetic particle modes evolves during non-linear chirping phases in tokamak plasmas, revealing different behaviors for BAEs and EGAMs.

Contribution

It provides the first detailed analysis of the mode structure changes of BAEs and EGAMs during non-linear chirping in tokamaks, highlighting the dependence on plasma parameters and particle resonance.

Findings

Down-chirping BAEs show minimal structural change, likely due to their nature as normal modes.

Upward chirping EGAMs exhibit shrinkage of mode structure, linked to resonance shifts.

Radial structure behavior depends on mode type and resonance dynamics.

Abstract

Alfv\'en eigenmodes (AEs) and energetic particle modes (EPMs) are often excited by energetic particles (EPs) in tokamak plasmas. One of the main open questions concerning EP driven instabilities is the non-linear evolution of the mode structure. The aim of the present paper is to investigate the properties of beta-induced AEs (BAEs) and EP driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated ASDEX Upgrade (AUG) discharges. This paper focuses on the changes in the mode structure of BAEs/EGAMs during the non-linear chirping phase. Our investigation has shown that in case of the observed down-chirping BAEs the changes in the radial structure are smaller than the uncertainty of our measurement. This behaviour is most probably the consequence of that BAEs are normal modes, thus their radial structure strongly depends on the background plasma parameters rather than on the EP distribution. In the case of rapidly upward chirping EGAMs the analysis consistently shows shrinkage of the mode structure. The proposed explanation is that the resonance in the velocity space moves towards more passing particles which have narrower orbit widths.